The problem I'm having is my 2009 i7 iMac refuses to give me 1000baseT (Gigabit) ethernet. It auto-negotiates to Fast Ethernet, despite everything in my network chain offering Gigabit speeds. When I attempt to force the connection to 1000baseT, it returns an error saying "Cable unplugged." I have no trouble seeing 150MB/s speeds on Wi-Fi.

If I unplug the Ethernet cable and plug it into my work laptop, I'm seeing 166MB/s speeds, meaning it's the iMac, not the connection, that troubles me.

Can anyone assist me? Numerous previous questions to the Support Communities for this exact same issue always lead to the same responses (i.e., "Are you sure your router/modem/cabling can handle it?").

If I unplug the Ethernet cable and plug it into my work laptop, I'm seeing 166MB/s speeds, meaning it's the iMac, not the connection, that troubles me.

Can anyone assist me? Numerous previous questions to the Support Communities for this exact same issue always lead to the same responses (i.e., "Are you sure your router/modem/cabling can handle it?").

Anyone have any ideas on this?

Click to expand...

Unfortunately, the cable really is the best place to start troubleshooting, regardless of whether or not it works in another computer. Have you tried another cable?

Router and access point: NetGear R6300v2 and NetGear R6300v1, respectively, although the access point is only used in WiFi mode; there are no ethernet cables attached to the R6300v1 except the one that connects it to the router.

Switch: Cisco SG100-16 16-Port Gigabit Switch. I wired my entire house with plenum-rated Cat6 cables so I have ethernet in every room, including the garage. Every wall port is wired to the Cisco switch with Tripp Lite cabling (see below); switch is connected to one of 4 gigabit ethernet ports on the R6300v2 router.

Tests were run with exactly the same equipment. I would run numerous speed tests on my iMac, remove the patch cable from the iMac, and plug the exact same cable into my work laptop. Numerous speed tests would then be run on the laptop.

iMac (2.8GHz i7 with 16GB of RAM):

Laptop (2.3GHz i7 with 8GB of RAM):

As one can see, my iMac is giving me Fast Ethernet speeds. When I attempt to select Gigabit Ethernet speeds, the iMac returns an error, saying the cable is unplugged.

No. The iMac will NOT render 1000baseT (Gigabit) Ethernet. I do back-to-back testing and the iMac drops to Fast Ethernet every time. If you do an internet search you'll see numerous people have reported this issue but not one single one of the threads on the issue provides a solution. Either the original poster simply disappears or respondents blame peripheral equipment (routers, cables, etc) instead of the iMac.

----------

When the settings are like this, it provides (obviously) Fast Ethernet speeds.

But if I change the speed setting to 1000baseT, I lose connectivity with the following message (enclosed in red, below):

Did this issue only start since you installed Yosemite?
Or did you already have the same connection issue prior to that upgrade?

Click to expand...

It's impossible to know if the OS is the issue because I upgraded to a 160MB/s internet connection after I went to Yosemite. Prior to the upgrade to the 160MB/s speeds, I consistently saw ~94MB/s uploads. Anecdotal evidence over the years has led me to believe the "ceiling" is just a bit below ISP-advertised values--that is, one doesn't often encounter 100MB/s speeds when paying for 100MB/s internet. In other words, the upper value is somewhat asymptotic: It's approached but never obtained. That said, I didn't think seeing ~94MB/s speeds was unusual for a 100MB/s connection. Besides, those speeds are obtainable with 100baseTX (Fast Ethernet). I never had any per se reason to worry about 1000baseT settings prior to going greater than a 100MB/s connection.

But are you running a beta of Yosemite with no functional previous system available, in case you need to revert (?) (Just asking )

You COULD install a previous system on something as lowly as a USB flash drive. 16GB is a nice size to have around for that purpose. Back to School sales have those for less than $10 this year (My fav time of year to stock up on pocketable storage ) A flash drive doesn't give you fantastic performance. but would quickly show you about your ethernet connection.

But are you running a beta of Yosemite with no functional previous system available, in case you need to revert (?) (Just asking )

Click to expand...

Indeed, but there's nothing on this computer that's mission-critical. Material I deem important is saved in multiple locations in the cloud. I employ Crashplan backup, Dropbox, Bitcasa, and, yes, a thumb drive.

If you have verified that your network cable (and the network) will give Gbit connection on another computer, but your iMac will not:
You said that you can't change the network advanced settings to force a Gbit connection, and:
Make sure you have tried a different OS X system on your iMac. Boot to a full system (not 10.10) installed on an external drive (or wipe your HD and install some prior OS X version) - you don't need full system performance, but you CAN check if the Ethernet will connect at Gbit speed on a different OS X version) :
So, in the end, you have verified the network and your patch cable are known-good, and the system software is also known good, and you still can't get the Gbit speeds, then only one item remains:

1st - I would verify if the entire system works under Mavericks or a version other than Yosemite DP.
2nd - If your laptop is pulling 166MBytes per second then it is going faster than Gigabit itself. Which is only 125MBps or 1200Mbit/ps. Big B Bytes, little b bits.

Unless you can verify it doesn't work under a system it should work under then your chasing ghosts.

Sorry but the first place I check is cabling, since you said all of the cabling in your house is passing CAT6 verification then the next place I would look is software since software seems to be throwing the issues.

Also, unless you have an ethernet tester it can be easy to only get 100Mbps speed even from CAT6 if your connections aren't terminated properly.

Hello from Spain.
I have the same problem, with OSX Mavericks. I reinstalled 2 times the system but I can not give a Gigabit connection, only 100 BASE T.
I have 3 computer more and all connect to gygabit.
I tried 5e cable and cat6, but the connection does not function.

Just to chime in here - I have Yosemite on a 2009 I7 27" iMac running at 1000Mb/s without problems!

DeputyRob - you seem to have gone through similar problems in 2013 with your ethernet connectivity using Mavericks Beta - You seem to like the Beta challenge !!

It is either a hardware or a software issue.
To narrow it down - can you put another Mac on exactly the same ethernet plug and see if it works?
Or put your iMac right next to your switch and connect with a short cable.

If it does, you have narrowed it down to your iMac.

Do a clean install on an external disk to make sure it is not your current Yosemite Beta instal.
If that does not help its the iMac's hardware - happy motherboard replacement.

Do a clean install on an external disk to make sure it is not your current Yosemite Beta instal.
If that does not help its the iMac's hardware - happy motherboard replacement.

Click to expand...

I have, indeed, narrowed it down to my iMac, as other machines, including my work Dell laptop, have no problems reaching gigabit speeds. Given it's the iMac, I'm not sure if I'm ready to assume the card is bad, as it seems many other iMac users have been afflicted by the same issue. You could be 100% right on the mark but I find it hard to believe the ethernet card works fine with everything else EXCEPT gigabit. It seems to me that ethernet functionality would be totally lost if it was the ethernet card itself. Of course, I'm certainly no expert.

One update: I'm no longer running a beta version of Yosemite so I think we can rule out it's tied to an issue with non public-release software.

MacRumors attracts a broad audience
of both consumers and professionals interested in
the latest technologies and products. We also boast an active community focused on
purchasing decisions and technical aspects of the iPhone, iPod, iPad, and Mac platforms.